In a micromechanical capacitive converter for which a silicon microphone is an example, frequently an air-filled cavity with a small volume is present. In a microphone, this is for example an air-filled sensor capacity consisting of a sensitive membrane and a rigid counter electrode. Due to this small air volume, the enclosed air exerts a strong restoring force on the sensor membrane. The enclosed air causes a damping of the membrane deflection and reduces the sensitivity or bandwidth, respectively, of the sensor.
For increasing the bandwidth it is known to provide discharge facilities for air, wherein this is done by a perforation of the counter electrode in silicon microphones. By such a perforation, the air may escape from the capacitor gap, i.e. the cavity between the sensitive membrane and the rigid counter electrode.
Well-established commercial elecret microphones comprise geometries with dimensions so great that the rigidity of the air cushion is neglectable. These microphones have, however, not the advantages of a temperature-stable silicon microphone in mass production.
In micromechanically manufactured microphones, ones with electroplated counter-electrodes are known, wherein the counter-electrode is electroplated in the last step of the manufacturing process on the microchip. With regard to such microphones, reference is for example made to Kabir et al., High sensitivity acoustic transducers with p+ membranes and gold black-plate, Sensors and Actuators 78 (1999), pages 138-142; and J. Bergqvist, J. Gobet, Capacitive Microphone with surface micromachined backplate using electroplating technology, Journal of Micromechanical Systems, Vol. 3, No. 2, 1994. In manufacturing processes for such microphones the perforation openings may be selected so large that the acoustic resistance is very small and has no influence on the damping of the membrane deflection. Disadvantageous is the expensive process of electroplating.
From the prior art, further two-chip-microphones are known, in which the membrane and the counter electrode are respectively manufactured on separate wafers. The microphone capacity is then obtained by “bonding” the two wafers. With regard to such a technology, reference is made to W. Kühnel, Kapazitive Silizium-Mikrofone, Series 10, Informatik/Kommunikationstechnik, No. 202, Fortschrittsberichte, VDI, VDI-Verlag, 1992. Dissertation; J. Bergqvist, Finite-element modeling and characterization of a silicon condenser microphone with highly perforated backplate, Sensors and Actuators 39 (1993), pages 1991-2000; and T. Bourouina et al., A new condenser microphone with a p+ silicon membrane, Sensors and Actuators A, 1992, pages 149-152. Also with this type of microphone it is technologically possible to select sufficiently large diameters for the perforation openings of the counter-electrode. For cost reasons, however, one-chip solutions are preferred. In addition to that, with the two-chip microphones, the alignment of the two wafers to each other is problematic.
With the one-chip microphones, the counter-electrode is manufactured in an integrated way, i.e. only one wafer is required. The counter-electrode consists of one silicon substrate or is formed by deposition or epitaxy, respectively. Examples for such one-chip microphones are described in A. Torkkeli et al., Capacitive microphone with low-stress polysilicon membrane and high-stress polysilicon backplate, Physica Scripta, Vol. T79, 1999, pages 275-278; Kovacs et al., Fabrication of single-chip polysilicon condenser structures for microphone applications, J. Micromech. Miroeng. 5 (1995) pages 86-90; and Füldner et al., Silicon microphone with high sensitivity diaphragm using SOI substrate, Proceedings Eurosensors XIV, 1999, pages 217-220. In the manufacturing methods for those one-chip microphones it is generally required to close the generated perforation openings in the counter-electrode again for the following processing in order to balance the topology.
One manufacturing method for such one-chip microphones is known from WO 00/09440. In this manufacturing method, initially perforation openings are generated in an epitactic layer formed on a wafer. In the following, among others for generating a sacrificial layer an oxide deposition is performed on the front side of the epitaxy layer, so that on the one hand the perforation openings are closed and on the other hand a spacing layer whose thickness defines the later spacing between membrane and counter-electrode, is formed. On this layer, a silicon membrane with the required thickness is deposited then. After the required processing of the electronic devices, in the area of the perforation openings the wafer is etched from the backside up to the epitaxy layer. In the following, from the backside an etching of the oxide is performed for opening the perforation openings and the cavity between membrane and counter-electrode. One part of the sacrificial layer between membrane and epitaxy layer thus remains as a spacing layer between the membrane and the counter-electrode.
One disadvantage of this hitherto known manufacturing method for one-chip microphones is that the hole diameter in the counter-electrode may not be larger than twice the thickness of the layer deposited thereon, so that the perforation openings may still be securely closed when depositing the sacrificial layer with the desired thickness. This is disadvantageous in particular insofar as the width of the individual perforation openings may not be realized so large that the acoustic resistance and thus e.g. the top cut-off frequency of the microphone sensitivity may be optimized.